1. Field of the Invention
The present invention relates to a sheet feed device and an image forming apparatus, and more particularly, to a sheet feed device capable of separating and feeding sheets one by one and an image forming apparatus including the sheet feed device.
2. Description of the Related Art
Image forming apparatuses, such as copying machines, printers, facsimiles, or multi-functional apparatuses, have been conventionally known which each include a sheet feed device that separates and feeds sheets stacked in a sheet cassette, a sheet tray, or the like to an image forming portion one by one.
Such a sheet feed device includes a separating portion that separates sheets one by one. There are the following various type of separating portions. For example, an air separation type separating portion is known which loosens a stacked sheet by blowing air the stacked sheet, and then sucks and conveys the sheets one by one. A friction separation type separating portion is also known which separates and conveys the sheet using a friction force between a sheet feed roller and a friction member and a friction force between the sheets. Further, an inclination surface separation type separating portion is known which brings the front end of a sheet into contact with an inclination surface and separates the sheet.
In recent years, a miniature image forming apparatus has been placed and used on a desk in many offices and homes, and thus it is preferable to realize miniaturization and low cost of the image forming apparatus. Therefore, since a sheet feed device installed in an image forming apparatus is also necessarily miniaturized and manufactured at low cost, the above-described inclination surface separation type is suitable for the miniaturization and reducing cost. In general, an inclination surface separation type sheet feed device includes a sheet feed roller attached to a roller arm installed to be vertically rotatable and is configured such that the sheet feed roller is brought into contact with a topmost sheet and sends the sheet, and then slides the sheet to a separation wall to separate the sheet (see Japanese Patent Laid-Open No. 11-49388).
As illustrated in FIGS. 7A and 7B, an inclination surface separation type sheet feed device 100 is installed such that a roller arm 163 holding a sheet feed roller 162 is vertically rotatable about a rotation shaft 160. A topmost sheet S1 is fed by causing the sheet feed roller 162 to press the surface of the topmost sheet S1 among sheets S stacked in a fixed stacking portion 124. The topmost sheet S1 fed by the sheet feed roller 162 is slid to a separation wall 125, is separated by the friction of the separation wall 125, and is sent.
In the inclination surface separation type sheet feed device, when the number of stacked sheets S is large, an angle α formed between a line, which binds a rotation fulcrum of the roller arm 163 and a contact position of the sheet feed roller 162 with the topmost sheet S1, and a stacking surface of the fixed stacking portion 124 is small (see FIG. 7A). Conversely, the angle α increases, as the number of stacked sheets S is smaller (see FIG. 7B). On the assumption that F is a rotation force of the sheet feed roller 162, the abutting pressure (feeding pressure) of Fsinα for the topmost sheet S1 is applied from the sheet feed roller 162. Accordingly, an abutting pressure (feeding pressure) of the sheet feed roller 162 to the topmost sheet S1 is increased, as the angle α increases. In contrast, the abutting pressure (feeding pressure) is decreased, as the angle α decreases. Therefore, when the number of stacked sheets S is large, there is a concern that no sheet may be fed (feed failure) since the abutting pressure is low. Further, when the number of stacked sheet S is small, there is a concern that two or more sheets may be fed (double conveyance) since the abutting pressure is high.
A difference in the thickness of a sheet, for example, a difference in a basis weight has a large influence on whether the feed failure or the double conveyance occurs. When a thin sheet with a small basis weight is fed, a necessary abutting pressure is small. However, when a thick sheet with a large basis weight is fed, a necessary abutting pressure is large. In recent years, in order to reduce an environmental load, a thin sheet (for example, a sheet with a basis weight in the range of about 50 to about 60 (g/m2)) with a low basis weight is used in many cases. When a thin sheet is fed, a necessary abutting pressure is low. Therefore, when the number of stacked sheets is small, there is the concern that the double conveyance may occur.
It is desirable to provide a sheet feed device in which a sheet separation capability is improved.